Generally, it is necessary to meet strict conditions, such as corrosive resistance, cleanness, reliability, and versatility, for a pressure sensor for detecting the pressure of a fluid to be permitted as a sanitary pressure sensor that needs consideration to sanitation used in production sites for food, drugs, and the like.
For example, to ensure corrosive resistance, a sanitary pressure sensor needs to include a material having high corrosive resistance, such as stainless steel (SUS), ceramic, or titanium, in the wetted part in contact with a measurement target fluid (for example, liquid) for the pressure. In addition, to ensure cleanness, a sanitary pressure sensor needs to have a flash diaphragm structure that enables easy cleaning and high heat and impact resistance against steam cleaning. In addition, to ensure reliability, a sanitary pressure sensor needs to have a structure (oil free structure) that does not include encapsulant and a structure (high rigidity barrier) that makes it difficult to break a diaphragm.
Since materials and structures that can be used for a sanitary pressure sensor are restricted as compared with other pressure sensors as described above, it is difficult to improve the sensitivity. For example, to achieve a structure that makes it difficult to break a diaphragm, although the film thickness of the diaphragm needs to be increased (the aspect ratio of the diameter to the thickness of the diaphragm needs to be reduced), increase in the thickness of a diaphragm generally reduces the sensor sensitivity because the amount of deformation of a diaphragm becomes minute. Accordingly, a technique for accurately detecting minute deformation of a diaphragm is necessary for a sanitary pressure sensor.
For example, PTL 1 and PTL 2 disclose load conversion type pressure sensors that improve the sensitivity of the sensors by transmitting the displacement of only the center part of a diaphragm to a semiconductor chip (beam member) of Si, or the like, on which a strain gauge, including diffusion resistors, is formed and detecting changes in the resistance value of the diffusion resistors caused by the piezoresistive effect based on the strain of the semiconductor chip.
Specifically, in the conventional load conversion type pressure sensor disclosed in PTL 1 and PTL 2, the center part of a semiconductor chip that is rectangular in plan view is supported by the center part of a diaphragm and both ends of the semiconductor chip are fixed at a position that does not move substantially. For example, in PTL 1, the center of a strip semiconductor chip is supported by a rod-shaped member called a pivot at the center of the diaphragm and both ends in the longitudinal direction of the semiconductor chip are fixed to the thick-walled part formed at the outer peripheral edge of the diaphragm via an insulating base. In addition, in PTL 2, the center of a rectangular semiconductor chip is fixed to the center of a diaphragm and the both ends in the longitudinal direction of a semiconductor chip are fixed to a seat that does not move.